This invention is directed to reducing noise in the conversion of an analog signal to a digital video signal by using histograms of sampled images to reduce the amount of noise over a period of time.
It has become increasingly popular to use multimedia display systems to make presentations at business meetings, sales demonstrations, and classroom sessions. Most multimedia projection display systems receive analog video signals from a personal computer (PC). The video signals represent still, partial-, or full-motion display images of the type rendered by the PC. The analog video signals are converted into digital video signals to control a digitally-driven display object, such as a transmissive liquid crystal display (LCD) or a digital mirror device (DMD), to form the display images for projection onto a display screen.
Two common types of multimedia projection display systems are LCD projectors and LCD projection panels. An LCD projector includes a transmissive LCD, a light source, and projection optics to form and project display images in the manner described above. An LCD projection panel includes a similar transmissive LCD to form the display image, but operates with a conventional overhead projector (OHP) having a light source and projection optics, to project the display image onto a display screen. Examples of such LCD projectors and LCD projection panels are sold under the respective trademarks LITEPRO and PANELBOOK by In Focus Systems, Inc. of Wilsonville, Oreg., the assignee of the present invention.
One desirable feature for multimedia display systems is compatibility with the various analog video signal modes generated by various PC""s. These modes generally range from 640xc3x97480 to 1600xc3x971200 resolutions provided at image refresh rates of 60 to 100 Hz. The resolution expresses the number of horizontal and vertical pixel elements that can be turned on and off. Given the variety of resolution modes, multimedia display systems include an interface that converts analog video signals of various modes to digital video signals capable of controlling the LCD.
Analog video signals comprise an analog image data signal for each of the primary colors red, green and blue, and digital timing signals, which may include a pulsed horizontal synchronizing signal (Hsync) and a pulsed vertical synchronizing signal (Vsync) or a composite sync signal. The individual analog color signals are generated from bit data in a memory portion of the PC, using three digital-to-analog (D/A) converters, one for each of red, green, and blue. A complete image is typically displayed during a time interval known as a xe2x80x9cframe period.xe2x80x9d Each video frame is usually produced to have a central active video region surrounded by an inactive (xe2x80x9cblankedxe2x80x9d) margin. The resolution refers to only the pixels in the active video region. The state of each pixel, its color or shade of gray, for example, is described by several bits of data. The exact number of bits depends upon the desired number of colors or gray levels. Because of the large number of pixels and multiple bits required to specify the optical state of each pixel, a large amount of image data is required to characterize the image of each frame. For example, a typical liquid crystal display may have 480 rows and 640 columns that intersect to form a matrix of 307,200 pixels.
Because the LCD used in multimedia display systems require digital video signals, either the LCD or the system normally has an analog to digital (A/D) signal converter for converting the PC-generated analog video signals into a digital format suitable for driving the LCD. The A/D signal converter is usually combined with a phase-locked loop (PLL), which may comprise a phase comparator, a low-pass loop filter, and a voltage-controlled oscillator (VCO) formed in a loop to generate a feedback signal that locks into Hsync. In order to generate a selected multiple n of clock pulses for each period of Hsync, a divide-by-n counter is added to the feedback loop between the VCO output and the phase comparator.
The number n of individual pixel pulses per Hsync pulse may be set by reference to the resolution mode of the analog video source. To set the resolution mode, certain characteristics of the analog video signal, such as Hsync and Vsync may be used to refer to a mode look-up table stored in the display system CPU. The number n should be set to equal the number of pixel data components in each horizontal line of the scanned analog signal, including those active video data region and the blanked margin regions on either side of the active region. For example, for a screen resolution of 640xc3x97480, n may be set at about 800 to include the blanked regions on either side of the 640 pixel-wide active video data region. Thus, the pixel clock would sample the continuous stream of analog image data 800 times along each horizontal line of the frame.
FIG. 1 shows the desired relationship between the analog video data signal 1 and the pixel clock signal 4 is that the number n of pixel clocks 5 is set to establish a one-to-one relationship between pixel clock pulses 5 and pixel data components 2 of the analog data signal 1. This one-to-one relationship requires that the pixel clock signal frequency be equal to the analog video data signal frequency. Under this relationship, each pixel data component 2 of the analog signal is sampled by a single pixel clock pulse 5, which reads the instantaneous voltage value of the pixel data component so that it can be digitized. Since the pixel clock pulses 5 have xe2x80x9cjitterxe2x80x9d zones 6 at their leading and trailing edges, the clock pulses 5 should be registered with the centers of the pixel data components 2, so that the sampling is not randomly pushed by the jitter into the transition regions of the analog video signal. The stream of digitized values form the digital video data signal, which is addressed to the display object to appropriately set display object pixels at blank (black) or selected activated (non-black) status to replicate the image defined by the analog video signal.
Unfortunately, such A/D conversion is often imperfect due to errors in the pixel clock sampling of the analog signal. Such sampling imprecision gives rise to frequency (also known as xe2x80x9ctrackingxe2x80x9d) and xe2x80x9cphasexe2x80x9d errors, both of which may degrade the quality of the image.
Referring to the analog video signal 1 and pixel clock signal 4xe2x80x2 in FIG. 2, tracking error results from the number n of pixel clocks being improperly set. As discussed above, the number n of pixel clocks should be equal to the number of pixel data components 2 of each horizontal line of analog video data signal. In FIG. 2, the improper setting of n results in the pixel data components 2 not being sampled at a consistent point. For instance, n is set too large in clock signal 4xe2x80x2. The resulting crowding of the pixel clock pulses 5xe2x80x2 yields an additive leftward drift of the pixel clock pulses 5xe2x80x2 relative to the pixel clock data components 2 of the analog video data signal 1. Such drift causes sampling in the transition regions 3. For instance, as indicated by positional bracket A, the leading edges 7xe2x80x2 of the third through the sixth clock pulses 5xe2x80x2 sample in transition zones 3 of the analog video signal 1. Accordingly, the transition zone data will be erroneous and the image information from adjacent non-sampled pixel data components 2 will be missing from the digitized video signal. If n is erroneously set large enough, the pixel clock pulses may be so crowded that individual analog pixel data components 2 may be double-sampled. On the other hand, if n is set too small, the dispersion of the pixel clock pulses results in a rightward drift wherein sampling may also occur in the transition regions. In all of these cases, the erroneous sampling provides erroneous video signal data that may degrade the image quality.
Phase error may occur even if the pixel clock signal frequency equals the analog video data signal frequency. As shown in pixel clock signal 4xe2x80x3 in FIG. 3, the clock phase may be erroneously set such that every pixel clock pulse samples a transition region 3 of the analog video data signal. Leading edge jitter makes such phase error more likely, since if the jitter zones straddle the intersections 8 of the pixel data components 2 and transition regions 3 of the analog video data signal I, the voltage will be randomly sampled on either side of the intersection 8. In any case, phase error is undesirable in generating undesirable noise, or xe2x80x9csnowxe2x80x9d in the video image.
A current system for a projection display system is connected to a multimedia source of the PC type. The projection display system may include an image capture circuit that automatically eliminates phase and tracking error. A microcontroller, which is part of a display system CPU, controls the image capture circuit. The image capture circuit includes a programmable delay device, a PLL, a divide-by-n-counter, an A/D converter, and an ASIC (Application Specific Integrated Circuit) that contains an image edge detection circuit. The microcontroller controls the delay device and the counter to eliminate phase and tracking errors. A display object is connected to the output of the A/D converter. A window random access memory (WRAM) is connected between the ASIC and the display object.
The A/D converter samples (reads) the instantaneous voltage value of the analog video data signal at the leading edge of each of the pixel clocks, thereby generating a series of sampled data signal values. The A/D converter then quantizes the sampled values by matching each value to one of a series of preselected voltage amplitude levels, which have corresponding numerical values. These numerical values are then represented digitally and coded to establish 8-bit data for each of the colors red, green and blue. The three eight-bit color data signals are input through the three respective color data signal channels to the ASIC. At the display object, the coded color data signal set pixels at blank (black) or specific activated (non-black) status corresponding to the sampled voltage level.
The digital video data signals output from the image capture circuit are manipulated by the WRAM and display object control module to appropriately control the display object. Each frame is addressed to the WRAM where the frames are stored until they are addressed to the display object. Typically, the frames are addressed to the WRAM at a faster rate than they are addressed to the display object. For example, each frame may be addressed to the WRAM at 80 Hz and addressed to the display object at 60 Hz. Therefore, the WRAM must include enough capacity or memory to store a number of frames at once.
Such current systems are not optimum due to the fact that every pixel of each frame must be held within the WRAM for comparison with pixels of consecutive frames. The WRAM is expensive and adds to the cost of the projection system because of its necessary large storage capacity. Additionally, the WRAM takes up a large amount of board space.
It is an object of the present invention to reduce noise in a digitally sampled image without the use of an expensive frame memory to store the entire frame image.
Another object of the invention is to reduce noise in a digitally sampled image by using a histogram of a sampled image to reduce the relative noise level of a static image over a time period of several frames.
Another object of the invention is to compare histograms of consecutive frames of data to determine the relative measure of digital noise present in a static image.
In accordance with a preferred method of the present invention, a digital video signal is produced from an analog video signal including an analog video data signal that is operable to be raster scanned in lines across a CRT screen to form consecutive frames of video information. The raster scanning is controlled by use of a horizontal synchronizing signal (Hsync) that controls a line scan rate and a vertical synchronizing signal (Vsync) that controls a frame refresh rate to produce consecutive frames of video information. The digital video signal is produced by generating a pixel clock signal with pixel clocks for repetitively sampling instantaneous values of the analog video data signal and digitizing the active image width of the analog video data signal based on the pixel clock sampling.
A frame of data is sampled using a pixel clock signal. A histogram of that frame is constructed by counting the number of occurrences of a value of color or range of colors in the frame and storing the number. A second frame is sampled and a histogram is constructed for the second frame. A comparison is made between the histogram of the first frame and the histogram of the second frame. The difference in the number of color value changes between the first and second histograms gives the system controller a relative measure of the digital noise present in the static image of that phase setting. The pixel clock phase is then shifted and histograms of the first and second frames of that phase are constructed and compared to each other to determine the difference in the number of color value changes in that phase. The process of constructing and comparing histograms of the first and second frames for each phase is repeated over time. After the set number of comparisons are performed the pixel clock phase is adjusted to correspond to the phase having the least change of color values.
In accordance to other aspects of the present invention, apparatus are provided for carrying out the above and other methods.